I agree with the previous two posters. I'd run DiskWarrior on the drive, then copy off all of the data. Once you're in the clear with your data, zero out the drive and see how it goes afterwards. However, if you're running anything like LimeWire, Poisoned, or Acquisition, don't be surprised to see directory corruption as a recurring issue. LimeWire is the worst offender, but I've seen the other two doing damage recently.
If, after zeroing, you get spontaneous directory corruption without running apps that cause corruption, I'd look into RMAing the drive.